Trying to match impedance

The long and short of it is that almost any preamp would be fine using the unbalanced input of the amp. However, in that the balanced input is in fact 1.2k ohm, you'll need to ensure that your preamp choice in balanced mode is less than a hundred ohms ouput impdedence. That's not all that uncommon in ss designs, but almost unheard of in tube designs.

They're out there and if you buy one that has both balanced outputs and se outputs then the worse case scenario would be that you run your amp from its se inputs.

Thanks again Montytx for the valuable advise.

It looks to me as if the Threshold was built to meet the balanced line standard, which is low impedance. The input impedance is set low to reduce ringing in line matching transformers that drive the input.

There are not that many preamps that can drive that load, but they *are* out there. To my knowledge there are only 2 tube preamps made that are equal to the task without loosing low frequency response. There should be a fair number of SS units that can drive that though.